The present invention relates to vacuum film holders for large copy cameras such as graphic arts cameras, photomechanical cameras and the like. The conventional means of securing the film to be exposed is via a vacuum back having a plurality of apertures defined therein, as is typified by U.S. Pat. Nos. 1,945,481; 2,895,706; 3,107,078 and 3,759,613. These references are representative of a large number of prior art patents which disclose vacuum film holders for use in copy cameras or graphic arts cameras.
The present invention is distinguished from this group of prior art patents in that it eliminates the conventional vacuum back. The vacuum back of the prior art served both to retain the film in its proper position and to define the focal reference plane within the camera. In the present invention, the focal reference plane is defined by an optically transparent, rigid and planar platen member which receives the photographic material to be exposed. The sensitized material is secured against this platen member by unrolling a transparent flexible vacuum curtain across its planar surface. A vacuum channel with vacuum passageways is arranged around the perimeter of the optically transparent platen member to evacuate the air between the vacuum curtain and the backing member to enable atmospheric air pressure to force the vacuum curtain against the sensitized material and thereby retain the material at the desired focal plane in the camera.
The use of an optically transparent backing member is known from the prior art, with examples being disclosed in U.S. Pat. Nos. 3,028,786; 3,734,615 and 3,762,816. However, U.S. Pat. Nos. 3,734,615 and 3,762,816 to Joseph H. Wally, Jr., relate to a condenser lens and assembly for securing a piece of photographic film to be copied. This is in contrast to applicant's invention, which retains a sheet of photographic material which is to become the copy. In the Wally patents, the image originates at the film and passes through the transparent member, whereas in applicant's invention the aerial image originating at the copyboard passes through the transparent member to the photographic material. U.S. Pat. No. 3,028,786 to W. Wanielista, assigned to the assignee of the present application, discloses a glass insert for use in a frame defining a vacuum channel around the glass insert. However, neither this reference nor either of the Wally references disclose the use of a flexible vacuum curtain or the use of an engagement means to force said curtain into engagement with the transparent platen.
It is also known in the prior art to provide a mechanical device for applying a flexible halftone screen over the vacuum back of a copy camera. Examples of this type of device are disclosed in U.S. Pat. Nos. 3,068,748; 3,433,567; and 4,043,663. In each of these references a halftone screen is disposed over the film which is positioned immediately adjacent the vacuum back, and the air between the film and the halftone screen is evacuated through openings in the vacuum back arranged around the exterior edges of the film. In two of these references, U.S. Pat. Nos. 3,068,748 and 4,043,663, roller means are used to force the halftone screen into engagement with the film and vacuum back to thereby remove air trapped therebetween.
Applicant's invention may be contrasted with the above references in that the prior inventors used the conventional vacuum back referred to previously. Furthermore, the references do not disclose the use of a rigid transparent platen to provide precise film focal plane registration. The halftone screens are not the equivalent of applicant's vacuum curtain in that a halftone screen is not transparent and does not provide a diffusing surface for displaying a viewable aerial image. Finally, the halftone screen is arranged between the film emulsion and the camera lens, whereas applicant's vacuum curtain is arranged on the base side of the sensitized material.
U.S. Pat. No. 4,089,063, issued to Robert Jacobs on May 16, 1978, discloses a fast-acting vacuum support assembly using a movable, optically transparent, rigid backing member and a flexible plastic sheet. However, this reference also uses a conventional back plate assembly to provide film focal plane registration for the movable member. Before the exposure is made, the transparent backing member is moved into engagement with the film, via the platen motor. In the Jacobs invention the glass plate is used expressly ". . . as the support platen rather than a back plate . . ." so that ".an optically flat back plate is not required . . ."Applicant's invention, on the other hand, uses the transparent, rigid, planar platen member as the back plate to maintain the film in a precise focal plane. In the Jacobs invention, the flexible plastic sheet is opaque rather than transparent and does not provide a diffusing surface to display the aerial image thereon. Furthermore, there is no engagement means of the type disclosed in applicant's invention to remove air trapped between the flexible plastic sheet and the film. Finally, if one were to remove the back plate assembly of the Jacobs reference, there would be no fixed focal plane reference for the photographic image.